If you don't want to spend $80-$100 on a TBT app, get Navigon now, it works.
If you have GPS Signal issues, follow their FAQ about how to get better reception. Everyone (who I've either worked/talked with in person, or traded forum posts with) who has thoroughly and genuinely followed those steps has no problems with GPS reception with Navigon thereafter.
For the past two weeks, the 1 & 2 star reviews of Navigon has leveled off, and the 4 and 5 star reviews are continuing to grow. Navigon put the FAQ link on the App Store description.
All that said.....IMHO, TomTom 'may' turn out to have better features, but they are known to charge a premium for it. I'm going to follow their launch closely, at least with regards to using their Software App's functionality WITHOUT the cradle (if they offer it that way). I don't need the cradle's functionality (I'm all setup for in car over the stereo setup), and I don't need the external GPS chip (Navigon works 99.9% perfect for me). But, if their software can function better/prettier/more features than Navigon's v1.0, I WILL contemplate purchasing the TomTom software. But....that also depends on how Navigon reacts with upgrades to their software package. Unfortunately, most of these apps are then going to be in the $80-100 range (for good rounded out map coverage, and not just a country by country/state by state version).
If TomTom's app comes out and has a ho-hum release/feedback/feature set, (or Navigon reacts with similar upgrades because TomTom moves the GPS TBT sector on the iPhone), I'm still going to be quite happy in saving the $$ for being an early adopter of Navigon.
HTH.